This invention relates to method and system for the nondestructive examination of the structural integrity of metals, and more specifically to a nondestructive examination to determine the extent of embrittlement, fatigue or dislocations throughout a metal specimen using neutron activated positron annihilation.
Fatigue in metal generally occurs in four stages: (1) early fatigue damage, (2) fatigue crack initiation, (3) fatigue crack growth, and (4) fracture. Most structural metals are polycrystalline and thus consist of a large number of individual ordered crystals or grains. The early fatigue damage generally consists of dislocations, dislocation loops and vacant lattice sites, which have accumulated into slip bands of dislocations within the grains. Some grains are oriented such that the planes of easy slip are in the direction of the maximum applied shear stress. Slip occurs in ductile metals within these individual grains by the dislocations moving along the crystalline planes.
Initially only a few bands are present in a few grains. As the fatigue cycling continues, more slip bands are observed in the grains with planes of easy slip in the direction of maximum applied shear stress, and more grains with slip bands are observed. Additional fatigue cycling creates more slip bands and also causes the slip bands to thicken. Most of the slip bands are on the component surface or on grain boundary surfaces and are not deep, but some are deep and are called "persistent slip bands".
Microscopic fatigue cracks generally grow from the persistent slip bands which intersect the component surface or the grain boundaries in the plane of the maximum shear stress range. As cycling continues, the microscopic fatigue cracks tend to coalesce and grow along planes of maximum tensile stresses. Crack initiation occurs when a microscopic crack grows to a detectable size or when several microscopic cracks join and form a detectable crack.
Early fatigue damage in either crystal defects or microscopic cracks is not detectable by standard NDE techniques such as x-ray diffraction, ultrasonic, eddy-current, magnetic techniques, and microstructural examinations. These techniques are capable of detecting a crack only after it reaches a significant size, that is, crack initiation stage.
Positron annihilation is a method that employs positrons from a radioactive source such as .sup.22 Na, .sup.68 Ge, or .sup.58 Co, to detect the presence of changes in the materials' microstructure caused by irradiation, cyclic loads or thermal exposure. A positron is a charged particle equal in mass to an electron and having a positive charge equal in magnitude to the negative charge of the electron.
Upon injection into metal, positrons rapidly lose most of their kinetic energy by collisions with ions and free electrons. An energetic positron injected into a solid is slowed down to thermal energies within 10 ps (1 ps=10.sup.-12 s). Upon thermalization, the injected positron diffuses away from the point where it thermalized, until it finally annihilates with an electron. During this diffusion process, the positrons are repelled by positively charged nuclei and thus seek defects such as dislocations in the lattice sites, where the concentration of nuclei is lower. A thermalized positron has a typical mean velocity of approximate 10.sup.5 m/s. The balance between the diffusion rate (after thermalization) and the annihilation rate of thermalized positrons is such that on average each positron has time to diffuse just a few tens of a micrometer from its point of thermalization.
Typical lifetime and trial distance traveled by a thermalized positron before it annihilates with an electron are 200 ps and approximately 20 .mu.m, respectively. The distance (.about.20 .mu.m) traveled after thermalization encompasses about 10.sup.5 lattice sites, so there is a good chance that the position will encounter a defect and be trapped, even if the defects are present at quite small concentration (10 parts per million of defects ensures that on average there is one defect for every 10.sup.5 lattice sites).
Complete annihilation of both particles occurs when a positron encounters an electron and their mass is converted into pure energy in the form of two, or occasionally three, gamma rays. If the positron and the electron with which it annihilates were both at rest at the time of decay, the two gamma rays would be emitted in exactly opposite directions (180 degrees apart), in accordance with the principle of conservation of momentum. Each annihilation gamma ray would have an energy of 511 keV, the rest energy of an electron and of a positron. In fact however, nearly all the positrons are essentially at rest, but the electrons are not. The momentum of the electron determines the momentum of the annihilating pairs and causes the direction of the gamma rays to deviate from the nimial value of 180 degrees. Likewise, the energy of the annihilation gamma rays deviates slightly from 511 keV, depending on the momentum of the electron, because of the Doppler effect.
Although positron annihilation measurements have been successfully used in the laboratory to measure the fatigue of metal specimen materials, the technique has not been successfully utilized in field settings, such as nuclear power plants and in place structures. There are a number of reasons for this, including the fact that it is difficult to put a positron source and gamma ray detector inside a reactor pressure vessel or inside the primary coolant system piping. Also, postiron annihilation gamma rays are potentially subject to interference from radioactivity in or on the component to be examined.
Another reason that in-situ positron annihilation techniques have not been successful, in nuclear and nonnuclear environments, is that positron from .sup.22 Na or .sup.68 Ge sources only penetrate about 20 .mu.m or 170 .mu.m or less into steel. Therefore, conventional positron annihilation techniques are limited to near surface measurements and generally must be conducted under controlled laboratory conditions.
It is an aspect of the present invention to provide a nondestructive examination method having a neutron activated positron annihilation within a metal test specimen.
It is another aspect of the present invention to provide a nondestructive examination method utilizing data measured from neutron activated positron annihilation to determine embrittlement or fatigue within metal specimens.
It is still another aspect of the present invention to provide a positron annihilation method capable of nondestructively examining the internal (i.e., up to three and one half inches in steel) structure of a metal specimen.